Spectral measurements of optical signals are made with Optical Spectral Analyzers (OSA) or Power Monitoring Units (PMU) typically at a measurement resolution in the range 0.05 nm to 1 nm. High resolution measurements tend to lose a significant spectral portion of the signal being measured and low resolution measurements capture all the signal power but also a significant amount of noise. In communications networks comprising a number of optical signal channels, where optical channels do not overlap and noise is not significantly filtered, the common solution is to identify the noise floor and measure the noise power of the optical signal, and subtract the noise power in order to obtain the optical signal power, as described in ITU-T Recommendation G.697 “Optical monitoring for DWDM systems”. In current and next generation optical transport systems operating at high spectral efficiencies the noise floor is not measurable because it is either strongly filtered or not accessible due to spectral overlap of optical channel signals. One solution that has been proposed assumes that an optical signal exists in only one polarization axis and that optical noise is distributed over both polarization axes. This approach estimates the optical signal to noise ratio (OSNR) by measuring the noise component in the polarization axis orthogonal to that of the optical signal to thus obtain the noise component of the optical signal. This method intrinsically suffers from Polarization Dependent Loss and is not applicable to polarization multiplexed signal modulation formats.